Normally, portland cements are manufactured by burning an intimately blended mixture of calcareous and argillaceous raw materials to form a clinker and intergrinding the clinker with a small proportion of gypsum to a fine powder. The gypsum is conventionally employed to retard, and thus provide a measure of control over the setting time of the cement when combined with water.
Certain disadvantages are associated with the use of gypsum as a retarder of setting time, e.g., high water demand, particularly when the gypsum contains clay, which also interferes with the retarding characteristics; and a tendency to produce false set because of the formation of hemi-hydrate under the influence of heat developed during milling.
Patents which provide relevant background for the invention include the following. U.S. Pat. No. 4,066,469 describes ultra-rapid hardening cement compounds which do not contain gypsum. In these compounds, an alkali metal or ammonium acid salt is employed as an additive to control the setting rate. Additionally, the composition contains calcium lignosulfonate.
U.S. Pat. No. 3,619,221 describes the use of water-soluble salts of amines and formic acid to accelerate the setting times of cement compositions. Advantageously, the amine salts can be employed in combination with conventional set retarding agents.
U.S. Pat. No. 3,884,710 describes the conventional use of gypsum and at column 3, lines 66-68 it is indicated that gypsum may be used as a set regulator in conventional cements.
U.S. Pat. No. 3,960,582 describes a low-porosity free-flowing cement base which does not contain gypsum. The gypsum is replaced by a combination of alkali bicarbonate with a lignosulfonate of sulfonated lignin. The patent also describes the use of conventional water-reducer/set-retarding agents such as carbohydrates and hydroxy acids like sodium gluconate.
U.S. Pat. No. 1,848,934 describes addition of ground limestone to portland cement clinker to produce an inexpensive cement. The patent also describes adding retarders if it is desired to produce a slow setting cement.
U.S. Pat. No. 2,880,102 describes the use of mixtures of clay and ammonium salts of carboxylic acids as additives to cement compositions not generally containing gypsum. U.S. Pat. No. 3,144,347 discloses the use of amine salts of hydroxy acetic or lactic acid as cement additives. German DT-PS 950446 discloses the use of calcium formate in cement compositions as a replacement for gypsum. The use of calcium formate is also disclosed in DT-AS 1232861. U.S. Pat. No. 2,646,360 describes addition of ground limestone to cement compositions.